Aiming at an improved efficiency of communication capacity allocation, digital coherent optical communication generally uses a polarization multiplexing method, which uses a polarization state including mutually orthogonal light waves. A polarization multiplexing method can achieve doubling of frequency usage efficiency, i.e., doubling of multilevel modulation, as compared to a method using single polarization. Examples of modulation method that uses this polarization multiplexing method include modulation methods such as dual polarization-quadrature phase shift keying (DP-QPSK), which represents modulation bits using phase information in each polarization to achieve a multilevel modulation of 4 bits per symbol, and dual polarization-16 quadrature amplitude modulation (DP-16QAM), which represents modulation bits using amplitude information in addition to phase information of each polarization to achieve a multilevel modulation of 8 bits per symbol (e.g., Non-Patent Literature 1).
The mutually exclusive nature between increasing of the multilevel modulation and increasing of transmittable distance requires the multilevel modulation to be changeable at a fine granularity depending on the required transmission distance to efficiently allocate a communication capacity. As one modulation method that allows the multilevel modulation to be selected at a fine granularity, multi-dimensional modulation methods have been proposed. A conventional polarization-multiplexed signal is mapped four dimensionally, that is, transmission data is mapped to constellation points independently in a two-dimensional phase space for each of polarizations existing in two directions. In contrast, in a multi-dimensional modulation, a transmission symbol is mapped in a dimension higher than four formed by multiple time slots (TSs) in addition to two polarizations and to the phase space. Multi-dimensional modulation allows the multilevel modulation to be altered at a fine granularity, and can improve noise tolerance owing to a greater Euclidean distance between symbol points (e.g., Non-Patent Literature 2). In addition, a method is also proposed that provides a multilevel modulation of 2 bits per symbol, equivalent to that of dual polarization-binary phase shift keying (DP-BPSK), in eight-dimensional mapping using a pair of neighboring time slots and a QPSK symbol constellation (e.g., Patent Literature 1)